Traditionally, a cordless window covering system includes a headrail, a covering material, a bottom rail and a driving device, wherein the driving device is usually a spring box. The covering material is positioned between the headrail and the bottom rail, and the covering material can be collected or expanded below the headrail when the bottom rail ascends or descends respectively. When the weight force of the covering material and the bottom rail is balanced by friction force of the whole window covering system, the bottom rail can stop at a position to retain the level of light blockage of the covering material. However, the friction force of the whole window covering system is difficult to be controlled effectively comparing to the weight force of the covering material and the bottom rail. In addition, the closer the bottom rail ascends to the headrail, the more covering material accumulates on the bottom rail, and hence the heavier the overall weight of the bottom rail and the covering material. Therefore, it is likely that the bottom rail would more or less descend for a distance, which is undesired, from a desired retaining position. In such case, it is inconvenient and annoying to anyone operating the window covering system.